The present invention relates to mass filters, including quadrupole mass filters, and, more particularly, to an electrode assembly, such as a quadrupole, for a mass filter.
Mass filters are tools for analyzing the chemical composition of matter, for example by using electric fields to separate ionized particles by their mass-to-charge ratios. High filtering resolution has been achieved using quadrupole mass filters that include four parallel elongated electrodes, the cross sections of which approximate hyperbolic arcs in respective quadrants about a common origin. Opposing pairs of electrodes are electrically connected.
A radio-frequency power amplifier (RFPA) is typically employed to drive both pairs of electrodes. One pair is driven with a selected radio frequency (RF) signal summed with a positive direct current (DC) potential The other pair of electrodes is driven by an RF signal 180.degree. out of phase with that applied to the first pair, and is summed with a negative DC.
The RF field dominates the motion of relatively light ions, ejecting them from the functional center region of the quadrupole filter. The DC field dominates the relatively heavy ions, which are gradually attracted and absorbed by one of the electrodes of opposite conductivity. Ions of an appropriate intermediate weight can traverse a generally longitudinal trajectory through the quadrupole due to offsetting RF and DC effects.
Thus, by properly setting the RF and DC components of the field inside the quadrupole arrangement, any mass within the unit's operating range can be selected for detection and measurement. Thus a single setting can be used in a single ion measurement (SIM) mode. By contrast, in a scanning mode, the RF and DC components are swept in a properly coordinated fashion to yield the fragmentation spectrum of molecular species in a sample.
The theoretically ideal cross section for the four electrodes of a quadrupole mass filter is four hyperbolic curves extending in their respective quadrants to infinity. Generally, only the hyperbolic arcs near the origin are approximated. These arcs are typically implemented by grinding the desired shapes from solid metal, e.g. molybdenum or stainless steel, rods. The desired relative arrangement of the four ground rods is then maintained, for example, by harnesses of ceramic or other rigid, non-conductive material.
However, there are several disadvantages to this four rod implementation of a quadrupole filter, e.g., expense, weight, bulk, and vulnerability to misalignment. For example, grinding identical hyperbolic surfaces on four several-inch long molybdenum rods is costly both in terms of time and materials. Furthermore, only the hyperbolic surface is electrically useful. The bulk of the rods serves only limited functions such as providing rigidity. If the four rods in ceramic harnesses are jolted, misalignment can easily occur. Furthermore, this misalignment may be undetectable by an unaided eye, and yet unpredictably distort mass readings.
One approach to ameliorating some of these problems has been to encase quadrupole rods in a square tubular glass frame. The individual rods are conformed to the frame, which results in less mass and bulk. The glass frame also serves to maintain rigidity when forces are applied. However, it is not clear to what extent the minor theoretical advantages of this approach translate into practice. Furthermore, additional improvements in weight, size and reliability are still required. Finally, no significant cost savings is apparent in this approach.
A more dramatic alternative being considered is the use of glass quadrupoles. Such a quadrupole is disclosed in U.S. Pat. No. 3,328,146 to Hanlein. The structure of an electrode assembly is provided by an appropriately shaped glass tube which serves as a substrate for the quadrupole. The desired hyperbolic shapes of the electrodes can be achieved by conforming thin strips of metal to hyperbolic contours on the inner surface of a glass tube.
This approach provides greatly reduced size and weight due to the substitution of glass and thin strips of metal for the rods in the aforementioned approaches. Cost and labor is greatly reduced since glass can be economically obtained, and can be formed by vacuum formation over a mandrel. The costs and time involved in grinding metal rods is reduced to that required to form a reusable mandrel, as opposed to four quadrupoles per mass filter.
Glass tends to be less susceptible than quadrupole metals to small inelastic deformation, so that valid measurements are generally obtainable except when the structural integrity of the glass is breached. Damage to a glass quadrupole is more readily detected visually than damage to a metal quadrupole. Thus, there is less likelihood of a damaged glass quadrupole being operated under the impression that it is providing valid measurements.
While the conception of a glass quadrupole suggests some significant advantages, reduction to practice has taken a tortuous path. Whereas the metal quadrupole mass filter had the luxury of extended commercial development, the glass quadrupole is required to compete with a mature technology at the outset. The years of user feedback and resulting adjustments and tweakings are necessarily telescoped in the development of a glass quadrupole mass filter.
The glass quadrupole introduces new geometries as well as new materials. For example, while both metal and glass quadrupoles are to approximate a hyperbolic cross section with four curves asymptotically approaching conceptual x and y axes towards infinity, the approximations diverge from one another. In the case of the metal quadrupoles, the cross section comprises four isolated closed curves, one for each rod. In the glass quadrupole, the fundamental shape comprises truncated hyperbolas which are interconnected rather than isolated.
The ideal hyperbolic cross section is determined to provide a predetermined ideal electric field in a cylindrical region about an axis extending orthogonally through an origin defining the hyperbolas. Both the metal and glass quadrupole approximations differ from the ideal so as to introduce "non-idealities" in this region which are not easily susceptible to complete mathematical characterization. Through years of development, the non-idealities in the electric field introduced by the non-ideal characteristics of metal quadrupoles have been minimized through extensive experimentation.
It is necessary, then, to identify, accommodate, and/or compensate for the performance affecting peculiarities of the new materials and geometries introduced in connection with glass quadrupole mass filters. Thus, it is a primary objective of the present invention to provide a mass filter which provides the size, bulk and reliability advantages of such a filter, without sacrificing the performance of mature metal quadrupole mass filters. Concomitantly, it is an objective of the present invention to provide a method of manufacturing such a quartz quadrupole filter.